The increasing use of laser light beams in military and other applications has underscored the need for protective systems and, particularly eye wear for protecting individuals from retinal damage. One method for dealing with this problem is the use of a conventional color filter coating on a pair of goggles worn by the individual. However, due to the fact that such filters filter out a wide range of wavelengths, the outside world tends to undergo drastic coloration when viewed through such a shield. In addition, if protection from a number of wavelengths is desired, the use of a multilayer conventional color filter reduces the quality of the system from marginal to unacceptable. Accordingly, attempts have been made to provide filters having extremely narrow bandstop regions.
A more desirable alternative to such conventional color filters is the holographic notch filter. Such notch filters, generally, have the desirable characteristic that upon being exposed with light of a given wavelength, in the proper holographic configuration, a very narrow bandwidth effectively reflective surface (which is optically a diffractive surface) will be formed holographically. This reflective surface or "notch filter" comprises recorded interference patterns in a photosensitive material such as dichromated gelatin. Such a holographically exposed gelatin layer will exhibit reflective properties along a very narrow range of wavelengths substantially identical to the recording wavelengths.
Such filters, however, have a disadvantage which stems from the fact that the interference pattern recorded in the gelatin layer during the holographic exposure process comprises a plurality of planes. When light is incident on the gelatin layer at an angle identical to that at which the layer was exposed, the incident light ray will experience the same distance between the planes. On the other hand, if light is incident at another angle, it will experience a distance between the planes which is somewhat greater or smaller than the separation between the layers. The distance between path distances between the planes at different angles of incident light is governed by trignometric relationships. Because the path distance varies, a holographically constructed notch filter adapted for use at a given angle will exhibit the desired notch reflective characteristics at different wavelengths for different angles.
While such devices have a wide range of possible useful applications, the above mentioned characteristic poses a serious problem in the event that one wishes to protect a small area object, such as the human eye from stray laser radiation emanating from a wide range of points on the horizon. In particular, if such a shield is placed in front of the eye, light directly in front of an individual would be reflected for light of a narrow range of wavelengths. On the other hand, light beams to the far right or left of the individual field of view will be reflected only for light of a different wavelength. In the case of laser hazards, the wavelengths are limited to a few discrete values which would be applied to the eye at a wide range of angles. Hence, a device such as that described above has only limited applications.
In our U.S. patent application Ser. No. 525,116 filed Aug. 22, 1983, the disclosure of which is incorpoated by reference, we described a system which removes some of the disadvantages of such earlier proposed systems. In particular, we proposed the construction of a holographic notch filter having a spherical optical configuration but whose actual shape may be flat or some other shape. The actual shape may be, for example, selected to be similar to ordinary spectacles. The optical spherical surface recorded in the gelatin layer is selected to have a center of curvature substantially coincident, in the typical case for spectacles, with the center of rotation of the eye of an individual. Thus, all light having the desired wavelength and traveling along a path which includes the user's eye will be normal to the optical surface when it passes through the gelatin layer and be will reflected.
However a serious limitation of such systems is the fact that dichromated gelatin is not sensitive to the various laser hazard wavelengths against which one desires to protect.